Selecting the optimum pickup velocity for a pneumatic conveying system
Q: Can we save on our bottom line by selecting the optimum pickup velocity for our pneumatic conveying system?
In a pneumatic conveying system, gas (normally air) is compressed either by a blower or compressor and used to convey a material from one location to another. The most common method is dilute-phase pneumatic conveying, which conveys material at a high velocity (4,000 to 7,000 fpm) and low pressure (5 to 15 psi) in a conveying pipeline. Pneumatic conveying requires a minimum conveying velocity, which is the airflow velocity that will keep the material in suspension within a conveying pipeline. At the material feeding point, the airflow velocity is called the pickup velocity. The pickup velocity of a material is dependent on the particle size distribution, true density, cohesiveness, moisture content, permeability, and air retention property. Normally, this pickup velocity is determined experimentally in a test lab. However, in many cases, an operator is unaware of the material's optimum pickup velocity and will tend to select a higher pickup velocity to avoid pipe blockage. For example, the actual pickup velocity of a material is 4,000 fpm, whereas, the operator may use 5,000 fpm as pickup velocity if the operator has to convey the material without testing. The energy spent to increase 1,000 fpm more velocity is typically unnecessary. Because of this, a considerable amount of energy is wasted over time.
Let's take a look at a specific case to understand this concept better. A dilute-phase pneumatic conveying system is used to convey material from one process to another and is separated by a 300-foot distance. The pressure drop developed in the conveying system is 8 psi and the inlet cubic feet per minute (icfm) needed is 600 to achieve 4,000 fpm optimum pickup velocity. Assuming a 50-horsepower blower is used for this purpose, based on the blower curve, the energy consumption is 26 horsepower (brake). However, if the blower was operating at 800 icfm with a system pressure drop of 10 psi to achieve 5,000 fpm velocity, the energy consumption would be 40 horsepower (brake). The difference is 14 horsepower. If the blower is running 24 hours a day, 7 days a week for 1 year, the energy consumption would be 14*0.746*24*365 = 91,469 units. If the unit price is 10 cents, the energy expense would be 9,147 USD, which is the unnecessary expense. If the same operation is running for 5 years, the loss would be 5*9,147 USD = 45,735 USD.
If the plant manager understood the science of bulk solids handling through education and training, he or she would have tested the material in a test lab to find optimum pickup velocity. The test expense is usually in the range of 3,000 to 5,000 USD. If he runs the operation at 4,000 fpm, he would save more than 40,000 USD over a period of 5 years.
Other issues caused by higher pickup velocity are particle degradation, product quality loss, and pipe elbow wear. The financial loss due to quality loss and elbow replacement cost should be added into the total economic loss. If this higher pickup velocity is rectified, a considerable economic gain can be achieved.